My friend Kevin Amter, one of the first art directors I ever worked with in this business, is a very successful freelancer.
So successful in fact, that he actually wrote a book about it. It's called Minds For Rent: How Advertising Creatives Freelance and it's got great tips and much practical advice for a career path that desperately needed a "How To" guide: as smart as many ad creatives are, they sort of lose it when it comes to things like negotiating compensation and schedules and they constantly wind up selling themselves short.
Now Kevin is a web-savvy guy (he's even got his own blog) and so the book is available online as a download. For the very reasonable price of US$10. (Which is probably about half a Euro these days or 5 Canadian dollars.)
»Download it here